public class Main {
    public static void main(String[] args) {

        System.out.println("Hello world!");
    }

        public ListNode reverseList(ListNode head) {
            ListNode cur = head, pre = null;
            while(cur != null) {
                ListNode tmp = cur.next; // 暂存后继节点 cur.next
                cur.next = pre;          // 修改 next 引用指向
                pre = cur;               // pre 暂存 cur
                cur = tmp;               // cur 访问下一节点
            }
            return pre;
        }

    static class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

        ListNode(int val) {
            this.val = val;
        }

        ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }
}